Sunny and Draco hate having their mouths opened for them. When I had to dose them with Panacur, I had to wrap them in towels (1 @ a time, of course), hold their upper jaw with one hand, use the other hand to pinch the skin under their jaw, while holding the needleless syringe in one pinkie, then pry their mouths open and put in the Pancur, at the same time trying not to squirt the liquid in too quickly. Hard to do with only 2 hands! I'm so glad I had a knowledgeable vet show me how to open their mouths, because there's no way I could have figured it out on my own. Draco got pretty good about it, but Sunny still hissed and bearded during the entire process.
I don't have any advice for you about opening tough guy's mouth, especially since his is so small. Just remember to be really gentle, as you would with any baby, and remember that getting nutrients into him is crucial to keeping him alive.
I'm sorry you had such a bad experience with your vet. It sounds like you didn't get nearly as much info as you needed. Definitely call them up yourself tomorrow and ask to get put through to the vet that your sister brought tough guy to, and make sure that you get all of your questions answered. No question is too small, and you have a right to the vet's expertise, especially since questions were left unanswered from this past visit. Before you call, make a list of all of your questions, during the phone call be friendly but firm, and make sure that you understand everything before you hang up. Most vets actually become more responsive when they see how much of an interest you have in your pet.
I'd also strongly recommend pushing to have a fecal sample examined. It shouldn't cost too much to process, especially since the vet's office already has info on your pet. My vet charges ~$80 for a fecal for an animal that hasn't been seen by the vet clinic before, but that price drops to $26 for an established patient.
Good luck, and I hope your little guy starts eating some more.
